Growth of transformed cells is characterised by loss of anchorage dependence, changes in cytoskeletal organisation and alterations in responses to normal growth modulators. In the present study the intracellular matrix systems were compared in cell lines derived from normal canine prostatic epithelia (CAPE) and from transformed epithelia (CPA) isolated from spontaneous canine prostatic adenocarcinoma and three human prostatic carcinomas (Du-145, LNCaP and PC-3). All cell lines were maintained in monolayer cell culture. Detergent extraction of these cells gave 4 subcellular fractions with differing protein composition, cytosol, cytoskeletion, chromatin and nuclear matrix fractions. A number of qualitative and quantitative differences in protein expression have been demonstrated using 1D and 2D PAGE analysed by immunochemical analyses of Western blots and in vivo labelling with [35S] methionine. In particular, differences in the expression of cytokeratin subclasses were demonstrated. Cellular contents of microtubule (α and β tubulin) and microfilament (actin) proteins were comparable. Unknown proteins, the relative expression of which were related to normal and neoplastic phenotypes, were also identified. Preliminary investigations into the differences in the levels of protein phosphorylation and in response to EGF on transformation were also carried out. The results obtained in this study are discussed with reference to the possible role of the cytoskeleton in growth control of normal and transformed cells.